An imaging apparatus is a device that converts a narrow "scan line" portion of an image of an object to machine-readable data. The machine-readable data generated by the imaging apparatus is referred to herein as, "image data." The process of converting a scan line portion of an image of an object to image data is known in the art as, "imaging" or, "scanning" the object. The imaging apparatus may be electrically connected to a processor that analyzes the image data. Based on the image data, the processor may be able to identify the object being imaged. An example of an imaging apparatus is a bar code reader. Bar code readers are well known in the art and image bar codes to identify the objects to which the bar codes are affixed. An imaging apparatus in the form of a bar code reader may, for example, be used in an automated media exchanger to identify the media located in the automated media exchanger.
The scan line portion of the object that is imaged by the imaging apparatus is created by reflecting light from the object. Reflective areas of the object will reflect more light than nonreflective areas of the object. Areas of the object that are reflective will, thus, correspond to areas in the scan line that have high intensities of light. Likewise, areas of the object that are relatively nonreflective will correspond to areas in the scan line that have low intensities of light. The scan line portion of the object may, thus, represent the intensities of light reflected from the object.
An imaging apparatus typically comprises a lens and a photosensor. Both the lens and the photosensor may be located in a housing. The housing may have an opening to allow light to pass from the object being imaged, into the housing, and to the photosensor. The lens is located in a light path between the opening and the photosensor and serves to focus an image of the object being imaged onto the photosensor. The photosensor images the object by converting a scan line portion of the image of the object to image data.
The photosensor typically has a linear array of photodetector elements (hereinafter referred to simply as photodetectors). The photodetectors may be spaced a predetermined distance from the centerline of one photodetector to the centerline of an adjacent photodetector. The individual photodetectors output voltages corresponding to the intensity of light they receive, e.g., a high intensity of light may correspond to a high voltage and a low intensity of light may correspond to a low voltage. As previously set forth, the scan line portion of the object being imaged by the photosensor may have areas of high and low light intensity. Therefore, the areas of high light intensity in the scan line may correspond to groups of photodetectors that output relatively high voltages. Likewise, the areas of low light intensity in the scan line may correspond to groups of photodetectors that output relatively low voltages. The image data output by the photosensor is the cumulation of voltage outputs from the photodetectors.
The photosensor may be electrically connected to a processor. The processor may analyze the image data from the photosensor and may store the image data in a data storage device. In analyzing the image data, the processor may determine the relative intensities of the light received by the photodetectors based on the voltage outputs of the photodetectors. The processor may also determine the lengths of the areas of high and low light intensity of the image of the scan line portion of the object received by the photosensor. Determining the lengths of high and low light intensities in the image may be accomplished by counting the number of successive photodetectors that output high or low voltages and multiplying this number by the centerline spacing between the photodetectors. However, the processor is unable to determine the actual lengths of the scan line portions of the object that generated these areas of high and low light intensity unless the magnification of the imaging apparatus is known.
The lens typically reduces the size of the image of the object focused onto the photosensor from the actual size of the object. This reduction in the size of the image of the object relative to the actual size of the object is referred to herein as the magnification of the imaging apparatus and is designated by the letter (M). As an example of the image reduction, an imaging apparatus may have a photosensor that is approximately three centimeters in length. The image of a scan line portion of an object focused onto the photosensor may, thus, be approximately three centimeters long. However, the scan line portion of the object from which the image was created may have a length of approximately 24 centimeters. Therefore, the imaging apparatus may have a magnification of approximately 1:8 or 0.125.
The magnification of the imaging apparatus is primarily dependent on three variables: the object distance, the image distance, and the focal length of the lens. The object distance is the distance between the object being imaged and the lens. The image distance is the distance between the lens and the photosensor. The focal length of the lens depends on the shape of the lens in addition to other optical characteristics of the lens. For example, the lens may comprise a series of individual lenses, thus, the focal length of the lens will depend on the interaction between the focal lengths of the individual lenses. The object distance varies depending on the location of the lens relative to the location of the object being imaged and the image distance varies depending on the location of the lens relative to the photosensor. The image distance is generally much smaller than the object distance, therefore, a small deviation in the location of the lens will have a much greater impact on the image distance than on the object distance. The image distance is, thus, typically the most significant variable affecting the magnification of the imaging apparatus. Small deviations in the object distance typically have negligible effects on the magnification of the imaging apparatus.
As stated above, the magnification of the imaging apparatus must be known in order for the processor to have the ability to determine the lengths of scan line portions of objects being imaged. The magnification of the imaging apparatus may be difficult to accurately determine given the sensitivity of the magnification to small deviations in the image distance. However, if the lens were able to be precisely located relative to the photosensor, the magnification of the imaging apparatus could be accurately determined.
During the manufacture of an imaging apparatus, the lens may be secured to the housing so as to be located a precise image distance from the photosensor. This precise placement of the lens relative to the photosensor may require that a mounting structure be provided for the lens that permits the lens to be secured to the housing at a predetermined and precise image distance from the photosensor. However, the mounting structure may add expense and complexity to the imaging apparatus. In addition, the process of precisely locating the lens a predetermined image distance from the photosensor may add additional expense and manufacturing time to the imaging apparatus. Furthermore, the process of precisely locating the lens may be subject to errors, which may lead to the imprecise placement of the lens relative to the photosensor. One example of this problem occurs if an assembler secures the lens at an incorrect image distance. This will cause errors when the imaging apparatus attempts to determine the scan line length of an object.
As an alternative to precisely locating the lens relative to the photosensor, the lens may be secured in the imaging apparatus at an unknown image distance from the photosensor. The image distance between the lens and the photosensor may then be physically measured. The measured image distance may then be input to the processor and permanently stored by the processor. The processor may use the measured image distance to calculate the lengths of scan line portions of objects being imaged. However, the process of measuring the image distance may add additional expense and manufacturing time to the imaging apparatus. Errors may also occur in measuring the image distance and inputting the image distance into the processor. Additionally, in the event the imaging apparatus is ever replaced, the image distance of the new imaging apparatus must be input to the processor. Errors in determining scan line lengths of objects may occur if a user neglects to input the new image distance into the processor or if the user inputs an incorrect image distance into the processor. The same errors may occur if the processor is replaced, a user may not correctly input the image distance of the existing imaging apparatus into the new processor.
Therefore, a need exists for an imaging apparatus that may be easily manufactured and calibrated to determine the image distance, and hence, the magnification of the imaging apparatus.